The application of polymeric coating, and in particular paint, can be useful for augmenting protection and aesthetics of a body, or for preparing the body for further processes such as application of further layer(s). In the following, mainly the case of the polymeric coating being a paint film is discussed, but the discussion also applies to bodies coated by other polymeric coatings.
Although the painting industry is more and more automatized by using paint robots, many paint films still show failures in the uniformity or visual appearance of the paint, or are painted on substrates which contain defects themselves. For these reasons, accurate quality control of paint films is an important part of the paint process. One important aspect of quality control is the measurement of the paint thickness, in order to ensure a uniform thickness within a predetermined tolerance range.
Car bodies are typically covered with a number of different layers which each have their own functionalities. Traditionally, each layer has to dry before a next layer can be sprayed on top. Since this is very time consuming, industrial paint lines utilize the more and more the so-called wet-on-wet technique. The idea of this technique is that a next layer is sprayed on a previous one while the latter is still relatively wet, which reduces the painting lead time between two layers drastically. Before curing, the automobile body then contains a multilayer of relatively wet layers on which early quality control is necessary.
Most state-of-the-art techniques for thickness determination determine just the total thickness of the entire paint film. Further, many of these techniques, such as acoustic and magnetic sensing, work only in contact mode. Furthermore, these techniques are paint unspecific, i.e. unable to account for specific properties of particular paint compositions, which results in large error bars, especially for thick layers and multilayers of different paints.
To overcome some of these limitations, recently methods based on THz radiation have been proposed. These THz based new methods allow non-contact measurements and thereby overcome an important drawback of the prior art. For example, JP 2004028618 A and EP 2213977 A1 describe respective methods for determining the thickness of a paint film using THz radiation. The thickness is obtained by subtraction of peak positions of a time-domain signal. The peak positions, together with a known group index of refraction of the paint, allow calculation of the thickness. However, the robustness of this method leaves room for improvement. Also, the method is only reliable for single layers of known paints with a known index of refraction, which is however typically not known. However, in industrial applications such as car body painting as cited above, typically a number of paint layers of various types are applied before the previously applied layer(s) have dried, which is not addressed properly by the known techniques.
Hence, known techniques for paint layer characterization leave room for improvement.